(1) Field of the Invention
The present disclosure pertains to an optical writing device performing optical writing onto a photoreceptor, and to an image forming device equipped therewith.
(2) Description of the Related Art
An image forming device, such as a printer, uses an optical writing unit writing an image to a photoreceptor with an optical beam emitted from light-emitting elements, which are provided as a plurality of tiny light-emitting elements aligned linearly.
As an example of such an optical writing unit, Japanese Patent Application Publication No. H8-216448 discloses a configuration having a plurality of light-emitting elements aligned in a single line in a main scanning direction, and a switch element provided for each of the light-emitting elements. Each of the switch elements switches between supplying and interrupting a drive current to the corresponding light-emitting element. In this configuration, the light-emitting elements are caused to emit light in alignment order by the respective switch elements consecutively being switched ON by a clock pulse signal.
The optical writing unit may be configured with the smallest spacing possible between neighboring light-emitting elements in the main scanning direction in order to achieve high resolution in the main scanning direction of the formed image. However, as described above, a configuration in which the light-emitting elements are aligned in a single line in the main scanning direction imposes a lower limit on the size of the spacing.
As such, as illustrated in FIG. 16, a configuration has been proposed in which a plurality of light-emitting elements 90 are divided into four lines 911 through 914 and aligned in zigzags along the main scanning direction. In the following, reference signs 90-1, 90-1, 90-3, . . . 90-N are used wherever necessary to distinguish among the light-emitting elements 90, in zigzag arrangement order. Optical beams from the light-emitting elements 90 in the light-emitting element arrays 911 through 914 perform writing of an image onto a photoreceptor drum 920, which is rotating in the direction indicated by arrow J.
Given the configuration in which N of the light-emitting elements 90 are arranged in zigzags, a greater quantity of the light-emitting elements 90 is arranged per unit length in the main scanning direction relative to a conventional array configuration.
Accordingly, a pitch interval in the main scanning direction of beam spots 921, produced by the optical beams emitted from the light-emitting elements 90, on the photoreceptor drum 920 is made smaller than in the conventional configuration disclosed in Japanese Patent Application Publication No. H8-216448. That is, the resolution in the main scanning direction is increased.
In this zigzag configuration, the arrangement positions of the light-emitting element arrays 911 through 914 are offset in a sub scanning direction. As a result, once the light-emitting elements 90-1 through 90-N are caused to emit light in each main scanning line using image data from an original image, as-is, a phenomenon occurs in that, for an expected image of one main scanning line from the original image on the photoreceptor drum 920, beam spots 922 (dashed lines) are illuminated at positions offset in the sub scanning direction due to the arrangement positions of the light-emitting element arrays 911 through 914, not producing a single main scanning line 931. In order to prevent this phenomenon, light emission timing may be offset by a time (hereinafter, Δt) corresponding to an offset Δd of the arrangement position in the sub scanning direction among neighboring light-emitting elements 90, for instance light-emitting element 90-2 relative to light-emitting element 90-1, light-emitting element 90-3 relative to light-emitting element 90-2, and so on.
Incidentally, this offset in light emission timing requires control to individually switch the light-emitting elements 90 belonging to each of the light-emitting element arrays between emitting light and being extinguished.
For example, with attention to light-emitting elements 90-1 through 90-4, during a period from writing start for one page to time Δt, light-emitting elements 90-2 through 90-4 must be forcibly extinguished while light-emitting element 90-1 is emitting light (state A). This extinguishing is required because otherwise, an image not present in the original image would be written.
Once time Δt has elapsed since writing start, there is a transition to state B, in which light-emitting element 90-2 emits light and light-emitting elements 90-3, 90-4 remain extinguished. Once time Δt has elapsed again, there is a transition to state C, in which light-emitting element 90-3 emits light and light-emitting element 90-4 remains extinguished. Once time Δt has elapsed yet again, there is a transition to state D, in which light-emitting element 90-4 emits light. The transition to each state must also occur at the end of the writing of the page. The same also applies to the other light-emitting elements 1-5 through 1-N.
For each transition, from state A to state B, from state B to state C, and so on, a different quantity of the light-emitting elements is forcibly extinguished. As such, executing the transition to each state requires the output of a signal to all of the light-emitting elements for maintaining the current state until the transition to the next state, while the quantity of light-emitting elements forcibly extinguished in each state changes over time.
Assembling a single output circuit with functions for managing the switching of these state transitions requires a complex configuration that incorporates multiple tiers of logic gates and so on.
Supposing that the total quantity of the light-emitting elements 90 is 16 000, and that each output circuit is responsible for one hundred of the light-emitting elements, then experience suggests that each output circuit requires 500 logic gates in order to manage the switching of the states as described above, increasing the overall circuit scale to 80 000 gates. Increasing the quantity of gates embedded in a semiconductor element increases the size of the semiconductor element, which leads to increased costs. This problem is not limited to image forming devices, and occurs generally in optical writing devices performing optical writing on a photoreceptor.